The separation and collection of particulates/aerosols from an air stream (or other fluid stream) is of concern in several contexts. In some cases, the goal may be to simply remove the particulates/aerosols from the fluid stream, thereby cleaning or purifying the fluid. Often it is desired to remove all particulates, regardless of composition, if the particulates are above a certain size. For example, automobile painting and the fabrication of silicon chips in clean rooms represent two situations in which all particulates large enough to result in an inferior product are desirably removed from the processing environment.
In other cases, particulates are collected for analysis to determine the type and concentration of such particulates/aerosols entrained in the fluid. For example, this technology may be employed in the detection of airborne biological or chemical warfare agents, the detection of biological contamination in confined spaces, such as aircraft or hospitals, or the detection of industrial pollutants (either in ambient fluid or in the effluent of smokestacks).
Much effort has been expended in the past in the detection and classification of particulates or aerosols in fluid streams. Various technologies have been employed to remove particles from an air stream to obtain a sample for analysis, including cyclones, impactors, and filters. Impactors generally direct a stream of fluid containing the particulates toward an impactor plate. Due to their inertia, the particulates striking the impactor plate are collected on its surface, while the fluid is deflected to the side. One potential problem with such impactors is that particulates may bounce off the impactor's collection surface upon impact and thus avoid collection. It would be desirable to provide improved impactors that minimize such an undesirable characteristic.
Virtual impactors have been used to sort particulates entrained in a flow of fluid based on particle size, as well as to increase the concentration of particles of a desired size. When used in connection with an impactor, greater control over the particles deposited on the impaction surface can be achieved. Virtual impactors may operate on a number of different principles, but all avoid actual “impact” as a means to separate particulates from a fluid in which the particulates are entrained, and rely on differences in particulate mass to induce inertial separation. Specifically, a particulate-laden fluid stream is directed toward a surface presenting an obstruction to the forward movement of the fluid stream. The surface preferably includes a void at the point where the particulates would normally impact the surface, to minimize actual impaction. When a major portion of the fluid stream changes direction to avoid the obstruction presented by the surface, fine particulates remain entrained in the deflected major portion of the fluid stream. Heavier or denser particulates, on the other hand, fail to change direction and remain in a minor portion of the fluid. The threshold particulate size that generally determines whether a particle will be entrained in the minor flow or the major flow is referred to as the cut size. For example, a virtual impactor having a cut size of 10μ will separate a flow of fluid into a major flow containing the majority of the fluid and the majority of particles smaller in size than 10μ, and a minor flow that includes a minor portion of the fluid, and a majority of the particles over 10μ. The concentration of particles over 10μ in the minor flow is thus substantially increased (due to the reduction in volume of the fluid in the minor flow). Concentration increases of about 10-fold are readily achievable, and virtual impactors in series can readily achieve concentration increases of about 100-fold. Some examples of virtual impactors can be found in U.S. Pat. Nos. 3,901,798; 4,670,135; 4,767,524; 5,425,802; and 5,533,406.
Once particulates have been collected (for example, deposited on an impaction surface), the particles can be analyzed to characterize the particles (i.e., biological or not, for example, using relatively simple optical techniques) or to more specifically identify the particles (which may require application of more sophisticated analytical techniques). The specific analytical technique employed will dictate whether the particles on the collection surface can be analyzed in place, or whether a liquid or gaseous sample needs to be obtained before analysis.
Accordingly, a need exists to develop a method and apparatus capable of providing samples collected from a fluid stream with minimal operator effort, and minimal chance of contamination. Such samples desirably should include a high concentration of particulates of a desired size. It would further be desirable to provide method and apparatus for removing collected particulates from an impact collection surface, and to transfer such particulates in a liquid or gaseous state to an appropriate analytical component for analysis. To facilitate adoption of such technology, it would be desirable for such apparatus to exhibit minimal operational costs and require minimal operator involvement once the apparatus has been properly configured.